Thin films of crystalline material are important in many fields of science and technology. In semiconductor electronics, there is considerable research and development directed toward obtaining high-quality semiconductor films on insulating substrates, especially silicon (Si) on insulator films of silicon dioxide (SiO.sub.2). Such structures are conventionally called SOI structures. One successful approach to SOI has been to form an SiO.sub.2 layer on a single crystal Si wafer and then to deposit Si by chemical vapor deposition (CVD) on the SiO.sub.2 insulator to form a polysilicon layer on the SiO.sub.2. Next, the polysilicon layer is formed into a single crystal layer by a number of techniques, such as Zone Melting and Recrystallization (ZMR). The top silicon film is then used to construct silicon devices which are isolated from each other by the underlying SiO.sub.2 insulator.
Devices formed on SOI substrates are capable of superior performance compared to devices formed on bulk Si substrates. This results from the fully dielectric isolation produced by the underlying SiO.sub.2 insulator of the SOI structure. Chief among the improved properties is higher packing density, higher radiation tolerance, faster switching speeds and CMOS circuits free of latch-up.
The present invention relates to improvements in the ZMR process used to form SOI substrates. In one version of the ZMR process, the recrystallization of the polysilicon layer is seeded or nucleated by the underlying single crystalline silicon wafer. A precursor structure comprising an insulating oxide layer is formed on the Si substrate. The oxide layer is then etched or scribed down to the underlying bulk single crystalline silicon wafer; thus exposing single crystalline Si seed regions. Then a polysilicon layer is deposited on the precursor structure. The polysilicon layer contacts the exposed Si seed regions. An optional capping layer is formed over the polysilicon.
In the past, the seed opening to the silicon wafer was in the shape of a relatively wide (about 10 microns or wider) continuous circular pattern about the periphery of the wafer near the edge. A stationary heater elevates the polysilicon layer to about 1000.degree. C.-1300.degree. C., i.e., near the melting point of the polysilicon. A moveable heating element is then translated past the structure to melt the polysilicon as the heating source moves along its path. Upon recrystallization, the polysilicon film is transformed to single or nearly single crystalline film seeded by the underlying bulk silicon wafer.
Several problems have been found with the above-described process. One is the serious tendency for the polysilicon melt to promote the mass flow of material away from the silicon seed, preventing uniform seeding. It would be very beneficial to avoid or reduce this tendency.
Another of the problems presently associated with the ZMR process is that the regions of polysilicon at which the seed openings are formed clearly cannot be used for device formation. Thus, portions of the wafer is wasted or non-productive. It would therefore be advantageous to minimize the area of such seed openings.
Also, without proper orientation of the seed openings, there is a substantial uncertainty in the seeding of the polysilicon as the melt traverses the structure. The polysilicon, which is not properly seeded during this uncertainty, also is not usable for device fabrication. Thus, it would be very advantageous to minimize this uncertainty in practice.